Damn shame that the Frenchies got rid of the pantalon rouge....so wonderful to view long lines of the chaps slogging up the road....poor blighters.... hacking along in "horizon bleu" must have played havoc with their morale.... eh what?....probably half the cause of the 1917 mutinies if truth be known!

And that 'Horizon Bleu' has about the same history. The material for making the dye with the colour indigo (trousers where red, but jacks where dark -jeans like- blue) ran out because of the war caused difficulties transporting it overseas. It mostly came with British transport ships, but they needed those themselves for they had to build and a complete army in 1914-1915. Anyway, the French decided not to give up the colour blue, but simply use less dye per uniform.
Thus the blue faded from dark to 'Bleu Horizon'.

- France did try several new & more modern uniforms in the 1900's and 1910's, sometime in a somewhat large scale.
Notably the Tenue Réséda:

But troops were booed on parades by the public and the new uniform were quite despised because of its german like feldgrau, because most people and politicians were quite fond of the red trousers...
Sadly, the minister of defence advocating the new uniforms died in an accident, allowing more conservative voices to take advantage and at first to change bakc the trousers from the reseda tenue to red, hence making the search for a new uniform useless.

What sadly french politicians & public did not understood at that time was the fact that changing the field uniform did not made it mandatory to make parade & sortie uniforms disappear or less elegant.
All the countries that switched to more modern uniforms at the time kept blazing parade & R&R uniforms, and the monotonous modern uniforms were only seen on the fields.

- Regarding the "red troosers = huge losses" legend.
I disagree. It might have been true in the first days of the war. But french tactics had more to do with the losses than the uniforms.
Keep in mind also that after a few days these were covered with dirt, faded in the sun and so on, so not so fiery.
Also, in combat, the coat's flaps were supposed to be shut over the trousers, limiting the "red" exposure. Can't say for sure if this regulation was quite followed or not, but considering august 1914 hotness and the fact that the french uniform was already to hot for summer operations, I think such regulations were seldom followed.

But even with the red trousers being a legend, a first change of uniform was made in early 1915, switching from red to dark blue. Then in 1916, introducing the helmet and a lighter blue. These changes were made to modernize the uniforms in light of earlier combat retex but some issues, like its heaviness weren't domestically adressed until... the Armée d'Armistice uniforms trials in 1941.

- Regarding the bleu horizon, I've heard divergent stories: some stated that the dye was made mixing blue, white & red, hence keeping a legacy of the old uniform. That's probably folklore.
Others stated that there was a shortage of dyes used to make the moutarde/khaki selected colors, hence the blue, whereas elite troops (armée d'Orient, some colonials, Foreign Legion) were issued moutarde/khaki uniforms that regulars units will have to wait for until the late 1920's - early 1930's.

Aside from the thread necromancy, the historiography of the Great War is so full of myths and legends that are held as common knowledge but collapse with some sensible interpretations of the actual events.

There's photos from 1913 around someplace that show the President of the Republic receiving the salute from a guard wearing steel helmets and low-visibility trial uniforms (at least so they seem to be low-vis in black and white).

So many myths accepted as "facts" is one of the things that make WW1 so interesting. Sifting through the propaganda and nationalist jingoism is all part of the fun.